Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks. OSPF is a link-state routing protocol that calls for the sending of link-state advertisements (LSAs) to all other routers within the same hierarchical area. Information on attached interfaces, metrics used, and other variables, is included in OSPF LSAs. As OSPF routers accumulate link-state information, they use algorithms that calculate the shortest path to various routers (network nodes). The largest entity within the hierarchy is an autonomous system (AS), which is a collection of networks under a common administration that share a common routing strategy. OSPF is an intra-AS (interior gateway) routing protocol, although it is capable of receiving routes from and sending routes to other ASs. An AS can be divided into a number of areas—groups of contiguous networks and attached hosts. Routers with multiple interfaces can participate in multiple areas. These routers, which are called Area Border Routers, maintain separate topological databases for each area. A topological database is essentially an overall picture of networks in relationship to routers. The topological database contains the collection of LSAs received from all routers in the same area. Because routers within the same area share the same information, they have identical topological databases.
The Shortest Path First (SPF) routing algorithm is the basis for OSPF operations. When a router using the SPF algorithm is powered up, it initializes its routing-protocol data structures and then waits for indications from lower-layer protocols that its interfaces are functional. After a router is assured that its interfaces are functioning, it uses the OSPF Hello protocol to acquire neighbors, which are routers with interfaces to a common network. The router sends hello packets to its neighbors and receives their hello packets. In addition to helping acquire neighbors, hello packets also act as “keepalives,” messages that let routers know that other routers are still functional. On multi-access networks (networks supporting more than two routers), the Hello protocol elects a designated router and a backup designated router. Among other things, the designated router is responsible for generating LSAs for the entire multi-access network. Designated routers allow a reduction in network traffic and in the size of the topological database.
When the topological databases of two neighboring routers are synchronized, the routers are said to be adjacent or collectively form an adjacency. Adjacencies control the distribution of routing-protocol packets, which are sent and received only on adjacencies. Each router periodically sends its LSAs to provide information on a router's adjacencies or to inform others when a router's state changes. By comparing established adjacencies to link states, failed routers can be detected quickly, and the network's topology can be altered appropriately. From the topological database generated from LSAs, each router calculates a shortest-path tree (SPT), with itself as root. The SPT, in turn, yields a routing table.
There are network deployments where a communication link between routers can be only unidirectional (UD) and there is a requirement for routing protocols to forward traffic over it. Routing protocols such as OSPF and ISIS can be extended in order to run directly over UD links.
One of the assumptions for support of a UD link from point A to point B is that there is an alternative path from B to A so that a control packet could follow the alternative path. If the alternative path is bidirectional then the flooding over UD link is unnecessary as the alternative path assures the flooding between A and B. However, this assumption may not always be true or upon link failure the topology could consist of an alternative path that contains unidirectional links. What is needed is a mechanism to be able to flood updates over UD links.
Opaque LSAs belong to a class of link-state advertisement used as a generalized mechanism to enable OSPF extensions. Opaque LSAs include typically an LSA header followed by a 32-bit aligned application-specific information field. Like other LSAs, an opaque LSA uses a LS-database distribution mechanism that allows and defines flooding information throughout the domain.